One common technique of manufacturing spring mattresses is the one known as the pocket technique. According to this technique, the springs are enclosed in pockets, i.e. they are enclosed individually by a cover material. Owing to this arrangement, the springs become relatively resilient individually, such that each spring may flex separately without affecting adjacent springs, which increases the user's comfort, since the load will be distributed more evenly across the surface that receives the load.
One problem inherent in this type of mattresses is, however, that they are a great deal more expensive and more complex to manufacture than many other types of spring mattresses, since the manufacturing thereof requires large quantities of material and, in addition, many different joining and mounting steps. Furthermore, these mattresses are comparatively heavy and compact owing to the large quantity of material required, which in turn tends to make the mattresses relatively rigid.
Consequently, there is a need for a mattress which is easier and/or less expensive to manufacture, but which at the same time offers a comfort at least equivalent to that obtained in prior-art pocket-spring mattresses.
A previous solution to this problem is described in WO 02/44077 by the same applicant. According to this solution, springs located adjacent to one another within a string are spaced apart by a separation distance, which makes the mattress less compact and, thus, cheaper and easier to manufacture. The separation of the springs is obtained by providing double weld seams between the springs, said weld seams being parallel to one another and arranged at a certain distance from one another. Alternatively, the separation is achieved by a weld seam that is extended in the lengthwise direction of the string. Although this mattress offers a number of advantages compared with prior art, the manufacture thereof is still relatively complicated and expensive.
Moreover, a common problem associated with basically all pocket-spring mattresses, including the one described above, is that excess cover material accumulates at the top and bottom of the strings. This is a natural consequence due to the fact that the covers used are not adapted to the shape of the springs, while the thickness of the springs is such that they stretch the pocket-shaped covers locally. The accumulation of material in the upper and/or lower part results in non-desirable layers in the mattress, called false lofts, which have poor load-bearing capacity and are experienced as uneven and uncomfortable by the user. Furthermore, the accumulation of material implies a great waste of material, which of course is inefficient in terms of costs.
Another common problem associated with basically all pocket-spring mattresses is that the mattress is relatively compact, which impairs the circulation of air through the mattress. This is a disadvantage in that the user may perceive the mattress as being unnecessarily warm, in particular against the parts of the body that are in direct contact with the mattress. Furthermore, a continuous circulation of air ensures a better bed hygiene, which maintains the freshness of the bed for a longer period of time. It is true that the increased space between the springs as described above with reference to WO 02/44077 boosts the aeration of the mattress, but there is also in this mattress a need for an increased circulation of air. Moreover, a problem with this type of mattress is that residual ends of material are formed at the end of the strings, which besides being a disadvantage in terms of cost also makes the mattress unsymmetrical and formless.
It is known from U.S. Pat. No. 1,455,847 to provide slits between adjacent strings within the same string. However, this mattress is of another type than those described above, in which the strings are interconnected in a different manner. Today, the conventional way of interconnecting strings to form pocket mattresses is to interconnect them by gluing between abutting surfaces. In the mattress described in U.S. Pat. No. 1,455,847, the strings are instead interconnected by threading a connecting strip through the slits of the strings according to different patterns, this being the only purpose for which the slits are provided. Furthermore, U.S. Pat. No. 5,319,815 and DE 40 26 502, for example, disclose mattresses in which cuts are provided in the strings. In these mattresses, cuts are provided from the bottom or the top of the strings. In addition, the strings in these mattresses are not arranged in parallel with one another but crosswise, the cuts being intended to join together the cross-laid strings. DE 44 35 771 discloses a mattress in which the strings are provided with cuts in the middle of the string. Also in this case, however, the mattresses referred to have cross-laid strings, and the purpose of the cuts is to allow strings to be threaded through other strings.